M. Viniegra

Gas phase hydrogenation of o-xylene over palladium catalysts

Abstract Gas phase hydrogenation of o -xylene over palladium supported catalysts was studied. Catalysts were characterized by hydrogen and carbon monoxide chemisorption and hydrogen-oxygen titrations. The supports used were silica, γ-alumina and silica-alumina. The products of the reaction were cis - and ( trans -1,2-dimethylcyclohexane and -1,2-dimethylcyclohexene. The selectivity was modified by temperature, metal dispersion, and acidity of the support, the production of the trans isomer increasing with an increase in each of these three factors. On the contrary, the presence of electron-donor molecules, such as pyridine, increased the selectivity to the cis isomer. The production off trans -1,2-dimethylcyclohexane may be adequately explained by the ‘roll-over’ of the intermediate species. It is proposed that the electron density of palladium is the determinant factor for the stereoselectivity of the catalysts.

The role of support in the catalytic hydrogenation of benzene over ruthenium catalysts

It is generally agreed that the hydrogenation of benzene over supported noble metal catalysts is a facile reaction which is independent of particle size. Previous benzene hydrogenation studies over supported Ru catalysts have been carried out using relatively noninteractive supports such as alumina and silica. Because of the possibility that more interactive supports such as magnesia or lanthana may modify the catalytic activity of supported Ru catalysts in the hydrogenation of benzene, the authors have extended previous studies to include magnesia, silica-alumina, and graphitic carbon as support materials. 12 references.

Probing RuPdSiO2 catalysts by gas phase o-xylene hydrogenation

Abstract RuPd SiO 2 catalysts were characterized by hydrogen chemisorption, XPS and TPD of CO, and their catalytic activity was obtained for the gas phase hydrogenation of o-xylene at atmospheric pressure. A small maximum in activity appeared in the bimetallic catalysts at temperatures at which palladium is inactive. The stereoselectivity of the bimetallic catalysts differed markedly from that of monometallic ruthenium. A ligand effect between surface Ru and Pd atoms is suggested to be responsible for the catalytic behaviour of the bimetallic catalysts.

Gas phase hydrogenation of m-xylene on palladium catalysts

Gas phase hydrogenation of m-xylene on palladium supported catalysts was studied and compared with the hydrogenation of o-xylene. The modification in the stereoselectivity of palladium due to an increase in the acidity of the support is explained in terms of an increased residence time of the reactive species on the active surface, leading to a product composition closer to that predicted by thermodynamics.AbstractГазофазное гидрирование м-ксилола на нанесенных палладиевых катализаторах исследовали и сравнивали с гидрированием о-ксилола. Изменения в стереоселективности палладия за счет повышения кислотности носителя объясняют повышенными временами местопребывания реактивных частиц на активной поверхности, приводя к продукту со строением, более близким к предсказанному термодинамикой.

Ruthenium-palladium catalysts: The effect of palladium on the catalytic behaviour of ruthenium

Abstract Ru-Pd/SiO2 catalysts were characterized by hydrogen chemisorption and their catalytic activity was obtained for the hydrogenation of benzene at atmospheric pressure. A synergistic effect appeared in the bimetallic catalysts with respect to monometalic ruthenium. This enhancement in the catalytic activity is explained in terms of the dilution of ruthenium ensembles by palladium. This increased the resistance to self poisoning of the active surface. It is also suggested that palladium serves as a source of hydrogen atoms for the reaction. Catalytic activity experiments performed in the presence of thiophene showed that this molecule inhibits the formation of undesired carbonaceous residues.

Gas phase hydrogenation of o-xylene on Ru supported catalysts

The gas phase hydrogenation of o-xylene on supported ruthenium catalysts has been studied. Metal dispersity, acidity of the support and carbonaceous residues modified the stereoselectivity of Ru. Following the “rollover” model for the reaction, the results are explained in terms of the residence time of the reactive species on the metal surface.

Specific behavior of β-zeolites upon the modification of the surface acidity by Cs and Li exchange

A study on the modification of the surface acidity of beta zeolite exchanged with Cs and Li was carried out by means of X-ray diffraction, nitrogen adsorption (BET), 27Al-MAS-NMR, (133Cs, 6Li)-MAS-NMR, surface paramagnetic shift (SUPAS) with adsorbed O2, FTIR of adsorbed pyridine and NH3-TPD. An inverse correlation between the amount of cations exchanged and the number of acid sites was verified, as well as a gradual decrease of the micropore surface area, i.e. a 38% negative variation for the fully exchanged solids ZβCs2 and ZβLi3. The 133Cs and 6Li (SUPAS)-MAS-NMR technique suggested that Cs cations were located in accessible sites, while Li cations were inaccessible and possibly located in cavities within the channels network. The crystallinity of the original beta zeolite was modified slightly after the ion exchange and no trace of amorphous material was detected by XRD and 27Al-MAS-NMR techniques. By means of NH3-TPD it was found that at similar cation molar concentrations the number of acid sites was about 4 times higher for Li-exchanged zeolites, therefore Cs showed a stronger neutralization power of the acid sites.

Dibenzothiophene Hydrodesulfurization Activity of MoS2 Supported in Sol–Gel ZrO2–TiO2 Mixed Oxides

Abstract In this work, we report the effect of support composition on the properties of MoS2 impregnated in sol–gel ZrO2–TiO2 mixed oxides as dibenzothiophene hydrodesulfurization catalyst. The supports calcined at 500°C were characterized by N2 physisorption and X-ray diffraction (electronic radial distribution function). The oxidic impregnated materials (2.8 Mo atoms/nm2) were sulfided at 400°C under a H2S/H2 stream. The sample impregnated on the equimolar support showed the highest activity per mass of catalysts whereas the one with TiO2 carrier was superior in a per mass of Mo basis. Marked differences in products selectivity were observed by TiO2 addition in the supports. The hydrodesulfurization route to partially hydrogenated compounds was favored over the mixed oxides-supported catalysts meanwhile the direct desulfurization (to biphenyl) was promoted on the ZrO2-supported solid. It is suggested that among other properties the dispersion and morphology of the MoS2 phase could influence that behavior.

n-heptane reforming over Pt supported on beta zeolite exchanged with Cs and Li cations

The physicochemical and catalytic properties of platinum supported on beta zeolite both acid and exchanged with Cs and Li cations were characterized by X-ray diffraction, NH3-TPD, FTIR of adsorbed pyridine and CO, DSC, mass spectrometry and hydrogen chemisorption. The n-heptane conversion was performed to assess the selectivity variations in the exchanged solids. A high metal dispersion was verified in all of the catalysts, varying from 54 up to 81%. The FTIR of adsorbed CO revealed a shift towards lower frequencies for Pt on exchanged zeolite with respect to Pt on the acid zeolite, indicating that the basic character of Cs and Li enhance the electron donation from the support to the metal. The acid–base properties of the exchanged materials influence their selectivity for the catalytic conversion of n-heptane at 663 K and 1 atm, showing that Cs promotes the aromatization reactions while Li promotes isomerization and cracking. The cracking activity followed the acid/metal ratio for all of the catalysts studied.

CO oxidation over CuO/ZrO2 catalysts: effect of loading and incorporation procedure of CuO

Abstract CuO/ZrO2 catalysts were prepared by in situ sol–gel and impregnation methods, with CuO concentrations of 1 and 3·6 mol.-% calcined at 400°C. The catalysts were characterised by S BET, XRD, diffuse reflectance ultraviolet-visible spectroscopy and temperature programmed reduction (H2). The catalytic activity was performed in the CO oxidation. The XRD, diffuse reflectance ultraviolet-visible spectroscopy and temperature programmed reduction results showed that the dispersion of Cu species depends on the method of preparation and on the copper loading. The CuO/ZrO2 catalysts prepared by impregnation procedure showed higher catalytic activity than those prepared by the in situ sol–gel method. This observation can be explained by the presence of small particles of CuO on the catalyst prepared by impregnation (higher dispersion), which does not occur in the catalysts obtained by the in situ sol–gel method due to an important amount of Cu2+ ions that were incorporated into the support network, which are considered catalytically inactive.